An X-ray CT apparatus, which includes an X-ray source which irradiates an object with X-rays and an X-ray detector which detects a dose of X-rays transmitted through the object as projection data, reconstructs a tomographic image of the object by using pieces of projection data from a plurality of angles obtained by rotating the X-ray source and the X-ray detector around the object, and displays the reconstructed tomographic image. An image displayed by the X-ray CT apparatus draws a shape of an organ of an object, and is used for image diagnosis.
The image quality of a reconstructed tomographic image changes depending on the magnitude of an inverse projection phase width which is an angle width of projection data used for reconstruction. In other words, as the inverse projection phase width increases, noise decreases, and as the inverse projection phase width decreases, noise increases.
PTL 1 discloses a method for reducing noise of an obtained tomographic image and also reducing noise unevenness. In other words, in PTL 1, an appropriate inverse projection phase width is calculated for each position of a pixel of a tomographic image, a view weight which is a weight coefficient multiplied by projection data and is a function of a view angle is calculated according to the inverse projection phase width, and a tomographic image is reconstructed by using projection data multiplied by the view weight.